In optical communication, a transmitting end must send an optical carrier to a modulator for modulation to load data information to the optical carrier for transmission. However, the modulator is generally formed by simple optical components and is vulnerable to external environment changes, so that an output modulated signal is affected. To lessen external impacts on the modulator, a direct current bias point of the modulator must change as the external environment changes. Therefore, the signal at the output end of the modulator must be detected.
The prior art provides a detection circuit that converts a sampling current into a sampling voltage by using a sampling resistor and feeds back the sampling voltage to a modulator. When the external environment changes, the change is fed back to the modulator, so that the modulator changes as the external environment changes. However, there is a huge difference between responsivities of different modulators, which results in such poor effects as a low locking speed of the modulator, horizontal shifts of a locking point, and a false locking point.
The prior art also provides another detection circuit, where multiple sampling resistors are arranged and are selected by using a switch. In this case, although the adaptability to the responsivity of the modulator is increased, the detection circuit is very complex and occupies a large Printed Circuit Board (PCB) area, which is disadvantageous for miniaturization of instruments.